This invention relates to cellular communications networks and is particularly but not exclusively related to fixed wireless access (FWA) applications.
Cellular communications networks are well known for both mobile and fixed subscribers. In cellular networks the coverage area is divided into cells, each cell is served by a base station or base site which allocates a frequency or group of frequencies which define communication channels between the subscriber and base station. The number of frequencies available to a cellular network is limited; so that the frequencies are re-used over different cells to make maximum use of these frequencies while at the same time maintaining an acceptable level of interference between neighbouring cells operating on the same frequency. One of the known methods for reducing interference is to divide each cell or base station into sectors, each sector being allocated a particular frequency or group of frequencies. In some conventional mobile subscriber sectored networks, frequencies are allocated such that the same frequency is given to corresponding similarly aligned sectors in each base site. While this reduces the level of a frequency interference from adjacent (first tier) cells, inference still occurs with cells beyond the adjacent cellxe2x80x94second, third tier and so on interference. Such arrangements are a particular problem in FWA networks where subscribers have high gain narrow beamed directional antennas, because the directional antenna is still aligned with co-frequency sectors in adjacent (first tier) and more distant (second, third . . . tier) cells.
There have been attempts to reduce co-frequency interference by varying the direction of sectors in some cells with respect to other cells such that sectors having the same frequency or frequency group are mis-aligned. By rotation of some cell sectors with respect to other cell sectors, direct co-frequency interference from first tier cells can be reduced.
It has also been known to change the polarisation of two adjacent co-channel sectors on an ad hoc basis to overcome severe and localised cases of co-channel interference.
Another method of reducing interference is to increase the number of frequencies or frequency groups allocated to each cell by increasing the number of sectors. In this way each sector is narrower and will therefore be less exposed to co-frequency sectors of adjacent cells, and for directional subscribers with suitable sector rotation the cell distance between direct interfering sectors can be increased. The number of frequencies or frequency groups allocated to each cell is known as the frequency reuse factor N which is a product of the base re-use factor Nb and the sector re-use Ns. The frequency re-use factor in GSM type mobile systems is typically 12-48. Generally the higher the frequency re-use factor N, the lower the co-frequency interference and hence the better the carrier to interference ratio (C/I). However the high frequency re-use factors typical in mobile systems reduces the capacity of the system in that less frequencies are available per base.
Most prior art systems are concerned with serving subscribers who are equipped with omnidirectional antennas such as mobile phones, which receive signals equally from all directions. The allocation of frequencies to base stations in these systems is therefore typically restricted to prevent strong unwanted interfering signals from first tier cells.
It is an object of the present invention to minimise co-frequency interference across a cellular communications network which serves directional subscribers.
In a first aspect the present invention provides a method of operating a cellular communications network which serves directional subscribers, the method comprising:
maintaining sectored cells in order to communicate wanted signals between a plurality of subscribers and base stations using a plurality of carrier frequencies;
maintaining directional mis-alignment and polarisation differences across the network between sectors carrying common frequencies in order to reduce co-frequency interference.
Preferably directional mis-alignment is maintained between sectors in first tier cells carrying common frequencies in order to reduce first tier co-frequency interference; and polarisation differences are maintained between sectors in second and higher tier cells carrying common frequencies in order to reduce second and higher tier co-frequency interference.
Alternatively, polarisation differences between co-frequency sectors can be maintained to reduce first tier interference and mis-alignment between co-frequency sectors can be maintained to reduce second tier interference. As a further alternative, sector mis-alignment can be used to reduce interference from all tiers with polarisation differences being used to reduce significant interferors.
Preferably the method comprises the further step of:
maintaining the cells in tessellating clusters such that co-frequency interference is reduced substantially uniformly across the network.
Preferably each said tessellating cluster comprises a number of repeating sector frequency patterns overlaid with a number of sector polarisation patterns.
Preferably each said tessellating cluster comprises:
a first sector frequency pattern comprising a number of cells and n repeated sector frequency patterns, wherein n is the number of sectors in each cell;
and wherein for each repeated sector frequency pattern, frequencies in sectors having subscribers directed towards base stations in said first sector frequency pattern are orthogonally polarised with respect to corresponding sectors in said first pattern.
Preferably said orthogonal polarisation difference is vertical and horizontal polarisation.
In a second aspect the present invention provides a cellular communications network which serves directional subscribers and comprises:
a plurality of base stations arranged to form sectored cells using a plurality of carrier frequencies;
wherein sectors carrying common frequencies are directionally mis-aligned and arranged such that the polarisation of said frequencies are varied with respect to each other across the network in order to reduce co-frequency interference.
Preferably sectors in first tier cells carrying common frequencies are directionally misaligned in order to reduce first tier co-frequency interference, and sectors in second and higher tier cells carrying common frequencies are arranged such that the polarisation of said frequencies are varied with respect to each other in order to reduce second and higher tier co-frequency interference.
Preferably the cells are arranged into tessellating clusters such that co-frequency interference is reduced substantially uniformly across the network.
Preferably each said tessellating cluster comprises a number of repeating sector frequency patterns overlaid with a number of sector polarisation patterns.
Preferably each said tessellating cluster comprises:
a first sector frequency pattern comprising a number of cells and n repeated sector frequency patterns, wherein n is the number of sectors in each cell;
and wherein for each repeated sector frequency pattern, frequencies in sectors having subscribers directed towards base stations in said first sector frequency pattern are orthogonally polarised with respect to corresponding sectors in said first pattern.
Preferably said orthogonal polarisation difference is vertical and horizontal polarisation.